In order to produce staff panels according to a dispersion method, a mixture of binding agents, reinforcement materials and optionally fillers is spread, mostly in several layers, upon a forming belt which moves continuously underneath dispersion devices and is subsequently compressed in a press. As binders gypsum or gypsum mixtures are used, e.g. hemihydrate, dihydrate, cement and additives, such as setting accelerants and/or retardants. As reinforcement materials, fibers, e.g. of waste paper, and as fillers e.g. perlite are used. The additives can first be mixed with the water and then added with the water to the plaster mixture.
During these process steps the gypsum has to be supplied with the water required for setting. It has been found that this is one of the most difficult tasks of the entire process.
In a process known from DE 27 51 466 and DE 27 51 473 the mixture is spread in a dry state and sprayed with water from above upstream of the press, whereby the water absorption is assisted by the vacuum applied under the permeable support belt. The dispersed layer can then absorb an excess of water, which is squeezed out in the press.
The disadvantages of this procedure include the need for a dewatering press. The resulting squeezed out water contains gypsum and there is a high residual wetness of the pressed plate which has to be dried.
By contrast in a so-called semidry process known from EP 0 153 588 only so much water is supplied as can be absorbed by the dry mass of gypsum, fibers and optional fillers, i.e. only so much water that during the pressing of the layer no water is squeezed out.
As is known from EP 0 153 588, the water is supplied in two fractions. The first fraction of water is supplied during the preparation of the spreadable mixture with a dry mix of gypsum and fibers. The second fraction of water is sprayed onto the individual layers, immediately after the spreading of the layer. Subsequently the wetted layer is compressed in a preliminary press and then compressed to its final density in a main press.
In order to improve this process it is known from DE 38 01 315 to let an upper screen belt run along in the presses during compression, for the purpose of venting. Further it is known from DE 39 06 009 to introduce the first water fraction with wetted and ground paper fibers. This way a mixer for the preparation of the dry mix can be eliminated. The spreadable mix is thereby produced by mixing the gypsum and the fibers provided with the first water fraction.
However in this process a considerable amount of water, the second water fraction, has to be sprayed. The spraying of spread layer or layers has a few disadvantages.
The loose, spread material can not absorb the water well enough. As a result in the upper areas there is excess water and corresponding lower areas with less water, which means that the wetting achieved by this spraying is uneven. The wetting evens out only during compression in the presses.
Since a water excess has an accelerating effect on the setting of the gypsum, the uneven wetting leads to different setting speeds in the spread, wetted layer.
There is also the danger that especially the bottom side of the first layer may receive too little water for the complete setting of the gypsum.
A further disadvantage is that devices for the spraying of the layers are arranged between the dispersion devices. In addition to the disadvantageous space requirements of the spraying devices, this arrangement requires that the various strata of the layer traverse different stretches until they reach the presses, thereby being wetted at different times. This results in differences in the onset of the setting process of the gypsum in the various strata.
Just like the different setting speeds due to uneven wetting in a layer, the different onset times of the setting in the various strata lead to weak areas in the finished plate caused by uneven setting. The control of the production process is made more difficult, especially since there is no possibility to monitor the setting process in the dispersed, wetted layer and to correct it.
DE-OS 40 25 797 describes a process wherein a portion of the second fraction of the water is supplied after compression by preliminary and main presses. The water is first applied by spraying to the bottom side of the plate and subsequently to its top side.
Subsequently the after-wetted plate is additionally compressed with a pressure which is not higher than that of the main press. In this case an additional press is needed. Besides only a small portion of the second water fraction can be supplied to a compressed plate during the additional wetting. A spraying of the spread material with the possibility of uneven wetting remains.